Two weeks ago, in his office at the medical bureau at Lourdes, Dr Patrick Theillier received a phone call. A 74-year-old woman who had made the pilgrimage to see the Pope when he visited the shrine in August had something extraordinary to report.

On the eve of the papal visit, while taking part in a ceremony for the benediction of the sick, she looked up and saw the sky. The glaucoma that had clouded her left eye - and which opthalmologists had judged inoperable - had, she said, receded. Her vision was fully restored.

"Very good," replied the phlegmatic Theillier. "Now go back to your opthalmologist and ask him if anything has really changed."

It's not that Theillier hears such stories every day. But perhaps once a week, 50 times a year, somebody claims to have been cured at Lourdes. The cure might be anything from relief from a minor ailment to the full-blown, biblical, get up from your bed and walk. The bureau's archives, which go back 120 years, record between 6,000 and 7,000 such events. Of those, 66 have been recognised by the Catholic church as miracles.

Lourdes, the site of a famous appearance by the Virgin Mary in 1858, has had a doctor since 1880, and Theillier was appointed to the post in 1998 by the bishop of Tarbes and Lourdes, Monsignor Jacques Perrier. It is his job to welcome pilgrims, doctors and carers from all over the world, and to oversee hygiene at the grotto, where the supposedly healing water springs from the ground.

His main duty, however, is to record and authenticate the unexplained cures.

A GP with 25 years experience and a deep respect for science, Theillier is also a practising Catholic. If he were not, he says, he could not do his job. Over the years, he has become hardened to fakers and publicity-seekers. But he continues to marvel at the medically inexplicable phenomena that pour into his office by post, phone or email.

It is his position at the boundary of science and faith, he says - as open to one as to the other - that has made him an object of fascination and mistrust in his own right. Constantly interrogated on the subject of miracles, he finally decided this year to sit down and write a book - a short conversation with young people in which he responds to their most frequently asked questions.

The book makes it clear that he does believe in miracles, but he goes to great lengths to explain exactly what he means by the word. "A miracle is something very precise," he says. "There must be a medical dimension and a spiritual dimension." And by way of illustration, he describes the case of Jean-Pierre Bély, the 66th and last miracle to have been recognised by the church, and the only one to have fallen to him to investigate.

Bély was 36 when in 1972 he started to suffer from neurological problems. Twelve years later he was diagnosed with multiple sclerosis and a year after that he was confined to a wheelchair. By 1987 he was receiving a full invalidity pension and, persuaded by friends, he agreed to go to Lourdes. During that visit, he began to notice feeling returning to his limbs, and a few days later his recovery was complete.

Theillier's role was to verify that Bély was in fact cured - which meant verifying that had been sick in the first place. He interrogated Bély and collected statements from numerous medical experts who could testify to the patient's previous state of health.

Theillier then presented his dossier to the international medical committee of Lourdes, a body comprising around 20 heads of European medical services - believers, agnostics and atheists among them. The committee first saw Bély in 1992, and demanded a second round of investigation. Key among the evidence that convinced them was the testimony of several psychiatrists that he was not suffering from hysteria - a psychological condition that manifests itself with physical symptoms. These can mimick those of MS but are nonetheless genuinely experienced by the patient.

Eleven years after Theillier first brought Bély's case to the committee's attention, they declared themselves satisfied that the cure was genuine and inexplicable in the light of medical knowledge. According to normal procedure in such cases, the file was then passed to Bély's home diocese, and after reviewing the facts of the case and the committee's decision, the church declared a miracle.

The process is long, says Theillier, but the first step is that medical science must recognise the cure as genuine and inexplicable. A case gets taken no further if it doesn't satisfy seven tough criteria.

The illness has to be fatal and one that can be diagnosed by physical examination. Psychiatric illness does not count, as its diagnosis relies on the patient's account, and the recovery has to be complete, sudden and permanent.

But the cure alone is not enough to persuade Theillier to take the case to the committee. "I believe the rigorous, scientific approach is very important," he says. "But that doesn't prevent me from listening to what the person experienced on a spiritual level." People tend to know the exact moment when they were cured, he says, and he listens closely to their account.

"As the priest was giving me unction on the forehead and hands, I had the impression that everything was turning around me," Bély writes in Theillier's book. "In a fraction of a second I lost all sense of time and space. God was coming to cure my heart. I was invaded by a powerful feeling of liberation and peace that I had never experienced before."

Theillier talks about the "marvels" he witnesses at Lourdes, but he is also realistic. "Listen," he says, "Ninety-nine per cent of the people who come here with a disease or a handicap leave with their disease or handicap. But they feel better here, almost inevitably, due to the climate of fraternity that exists here, the fact that they are being attended to, the love and tenderness that is lavished on them."

But what of the rest, the vanishing tumours, reversals of neurodegenerative disease, dissolving cataracts? The placebo effect explains a lot of it, says psychologist Irving Kirsch of the University of Plymouth - the idea that if you believe in a medical intervention enough, whether it is surgery or God's love, it can affect your health.

Over the years Kirsch and colleagues have conducted a series of studies showing the power of placebo. Five years ago, they published controversial findings that sugar pills could mimic 80% of the effects of anti-depressants such as Prozac, as long as people believed they were really taking the drug.

Perhaps even more dramatically, a study published in April by Cynthia McRae, Curt Freed and colleagues at the University of Colorado Health Sciences Center in Denver, showed that Parkinson's disease patients who believed they had undergone an operation to implant foetal tissue into their brains, but in reality had not, experienced significant improvements in their symptoms.

Despite the fact that people go to Lourdes in droves in the hope of being cured, Theillier denies that the placebo effect is at work there - much to Kirsch's surprise. But the head of the medical bureau has a different interpretation, which perhaps boils down to the same thing. "It is neither the grotto nor the waters," he says. "It is the concentration of people who are united in faith and share the same hope."

He also believes that the documented cases are just the tip of the iceberg. Many people don't report their cures, he says, partly because they know that by doing so they will expose themselves to interminable rounds of examination and questioning. "To be cured at Lourdes is a stress, a magnificent, positive stress," he says. Moreover, having experienced something so profound, so intimate and life-changing, many people prefer to keep it between themselves and God.

Whatever the explanation, some people do seem to get better at Lourdes, so why do scientists remain suspicious? Why do many hospitals still refuse to release their patients' medical records to Theillier?

"Placebo is a double-edged sword," says Kirsch. "Any treatment that someone has faith in might yield benefits. On the other hand, sometimes people are lured to abandon effective medical procedures for something that can have a detrimental effect."

He points to findings that placebos can trigger asthma attacks as well as relieve them.

Whether one believes in divine intervention, the power of placebo or mass belief, Theillier's attitude is that it is better to be there, guiding people to the medical treatments that could help them, and documenting the cures that happen spontaneously, so that science can ultimately benefit from those observations.

And yes, he has occasionally bathed in the waters, and offered up a prayer to Bernadette, patron saint of Lourdes - not for any particular malady; he just says it does him good.